In analytical neurochemistry, and particularly in <i>in</i><i> vivo</i> and tissue culture measurements, there are significant challenges due to the nature of the tissue and the important role of mass transport. We particularly are concerned with microdialysis and push-pull-type experiments, though some aspects of sensors will also be discussed. An important area for exploration is the understanding of the rates of extracellular chemical processes the measurement of which relies on controlled introduction of an active species and quantitative sampling of the process, e.g., an enzymatic degradation or an uptake. A Michaelis-Menten model is often adequate for this. However, in a push-pull type measurement, it is most straightforward to measure the result (uptake rate, enzymatic degradation rate) at a constant time which is defined by the push-pull conditions. In our case, we use electroosmotic flow, so in principle current could be used to explore different times, but the range of currents that can be used is rather small. Thus, one must deal with the integrated Michaelis-Menten equation with initial substrate as a variable and time as a constant (which is the opposite of the typical laboratory measurement). Another aspect of all such techniques is the effective diffusion coefficient and the changes in this quantity induced, e.g., by cells swelling (Nicholson, Sykova et al. have written masterfully on this topic). A simple method to assess these changes is required. We are developing a simple impedance method to determine regional variations in porosity. Finally, the combination of a tissue-property-sensitive diffusion rate and a concentration-dependent reaction rate leads to a difficult problem to solve in general. Thus, simulations are required to assess the impact of dilution/diffusion of an introduced substrate. We have found by simulation that the “measured” Vmax of a process occurring in tissue by electroosmotic push-pull measurements is approximately an order of magnitude higher than the actual Vmax if the laboratory concentration of substrate is used in the calculation (based on the integrated M-M equation). Thus significant simulation-based corrections to quantitative estimates of such properties are required.

Some considerations on limitations and opportunities in making measurements in living tissue

253rd National Meeting (2017)

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